2-chlorobutadiene-1,3 (“chloroprene”) is an important monomer used in the commercial manufacture of a number of synthetic chloroelastomers. Chloroprene may be produced from olefins using well-known multi-step processes that involve a series of chlorination and dehydrochlorination reactions of olefins. Such processes are disclosed in general in C. A. Stewart, Jr. et al., Chloroprene Polymers, in Encyclopedia of Polymer Science and Engineering, Vol. 3 (Second Ed), John Wiley and Sons (1965), 441-462. In one important commercial process butadiene is reacted with chlorine to form a mixture of 1,4-dichlorobutene-2 and 3,4-dichlorobutene-1. The mixture is then isomerized to increase the concentration of 3,4-dichlorobutene-1 and the resultant product is dehydrochlorinated, generally in the presence of aqueous alkali, to yield chloroprene monomer. Examples of such isomerization and dehydrochlorination processes are described, for example in U.S. Pat. Nos. 3,819,730; 4,827,059; 5,237,114; and 6,380,446.
The isomerization step may be conducted in the presence of copper salts and complexes, e.g. in the presence of a complex of a copper salt with a quaternary ammonium compound, as described in U.S. Pat. No. 3,819,730. An improvement to this process is disclosed in U.S. Pat. No. 4,827,059, wherein the isomerization is conducted in the presence of a hydroxylamine for the purpose of reducing formation of by-products. A disadvantage of using copper complex catalysts in the isomerization reaction is that when the isomerization is operated at low temperatures, e.g. below about 90° C., low conversions are achieved. Conversely, when the isomerization is conducted at high temperatures in the presence of copper complex catalysts, excessive amounts of by-products, for example, oligomerized and/or polymerized materials, are formed. This adversely affects efficiency and yield of the desired product.
It is also known to use other transition metal catalysts in isomerization reactions of dichlorobutenes. For example, the use of iron compounds and complexes as catalysts for the isomerization reactions of chlorobutenes is known. U.S. Pat. No. 2,242,084 discloses isomerization of 3,4-dichlorobutene-1 to 1,4-dichlorobutene-2 using iron chloride. U.S. Pat. No. 6,392,107 discloses the use of iron metallocene compounds having cyclopentadienyl and substituted cyclopentadienyl groups as catalysts for the isomerization reaction. In addition, the use of iron acetylacetonate to catalyze isomerization of dichlorobutenes is disclosed in G. Henrici-Olive and S. Olive, Kinetics and Mechanism of the Iron Catalyzed Positional Isomerization of Dichlorobutenes, J. Organometal. Chem., 29 (1971) 307-311. A disadvantage of both of these catalysts is that they are costly and contain ligands that are highly basic and easily protonated. Protonation results in some degree of catalyst decomposition which leads to deactivation and insolubility in the reaction media.
It would be desirable to have available alternative catalysts suitable for use in dichlorobutene isomerization that are capable of low byproduct formation and can be used at low levels but which are not associated with these disadvantages.